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From:   Aaron Thompson <dev@aaront.org>
To:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Valentin Schneider <vschneid@redhat.com>,
        linux-kernel@vger.kernel.org, Aaron Thompson <dev@aaront.org>
Subject: [PATCH] sched/clock: Fix local_clock() before sched_clock_init()
Date:   Thu,  6 Apr 2023 04:08:47 +0000
Message-Id: <20230406040847.569970-1-dev@aaront.org>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

Have local_clock() return sched_clock() if sched_clock_init() has not
yet run. sched_clock_cpu() has this check but it was not included in the
new noinstr implementation of local_clock().

The effect can be seen on x86 with CONFIG_PRINTK_TIME enabled, for
instance. scd->clock quickly reaches the value of TICK_NSEC and that
value is returned until sched_clock_init() runs.

dmesg without this patch:

    [    0.000000] kvm-clock: ...
    [    0.000002] kvm-clock: ...
    [    0.000672] clocksource: ...
    [    0.001000] tsc: ...
    [    0.001000] e820: ...
    [    0.001000] e820: ...
     ...
    [    0.001000] ..TIMER: ...
    [    0.001000] clocksource: ...
    [    0.378956] Calibrating delay loop ...
    [    0.379955] pid_max: ...

dmesg with this patch:

    [    0.000000] kvm-clock: ...
    [    0.000001] kvm-clock: ...
    [    0.000675] clocksource: ...
    [    0.002685] tsc: ...
    [    0.003331] e820: ...
    [    0.004190] e820: ...
     ...
    [    0.421939] ..TIMER: ...
    [    0.422842] clocksource: ...
    [    0.424582] Calibrating delay loop ...
    [    0.425580] pid_max: ...

Fixes: 776f22913b8e ("sched/clock: Make local_clock() noinstr")
Signed-off-by: Aaron Thompson <dev@aaront.org>
---
 kernel/sched/clock.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c
index 5732fa75ebab..b5cc2b53464d 100644
--- a/kernel/sched/clock.c
+++ b/kernel/sched/clock.c
@@ -300,6 +300,9 @@ noinstr u64 local_clock(void)
 	if (static_branch_likely(&__sched_clock_stable))
 		return sched_clock() + __sched_clock_offset;
 
+	if (!static_branch_likely(&sched_clock_running))
+		return sched_clock();
+
 	preempt_disable_notrace();
 	clock = sched_clock_local(this_scd());
 	preempt_enable_notrace();
-- 
2.39.2